studied and probably provides relatively accurate estimates of population size
(although precision, and especially accuracy, are not easily measured). Mapping
methods can also be usefully combined with nest finding, radio telemetry, mist
netting etc. in research projects. Mapping has seldom been used in the tropics,
largely because breeding is more asynchronous and many species have complex
social behaviors.
2.3.2 Transects
There are two types of transect most commonly used in bird surveying, line
transectsandpoint transects. The latter are often termed point counts. Both are
based on recording birds along a predefined route within a predefined survey
unit. In the case of line transects, bird recording occurs continually, whereas
for point transects, it occurs at regular intervals along the route and for a given
duration at each point. There are a number of variations on this theme where
birds are recorded to an exact distance (variable distance) or within bands (fixed
distance) from the transect point or line. The two methods can also be combined
within the same survey. While there are important differences between the line
and point transects, and choosing between them is an important decision in
survey design, there are also many practical and theoretical similarities.
Line and point transects are the preferred survey methods in many situations.
They are highly adaptable methods and can be used in terrestrial, freshwater, and
marine systems. They can be used to survey individual species, or groups of species.
They are efficient in terms of the quantity of data collected per unit of effort
expended, and for this reason they are particularly suited to monitoring projects.
Both can be used to examine bird–habitat relationships (though generally less well
than territory mapping), and both can be used to derive relative and absolute
measures of bird abundance. Transects can be usefully supplemented and, to some
degree, verified in combination with other count methods such as sound record-
ing, mist netting, and tape playback (e.g. Whitman et al. 1997; Haselmayer and
Quinn 2000).
There are a series of issues to consider when using transects in the field. The
recommended walking speed is particularly important for line transects, as are
the counting instructions for the observers. A further important consideration
is whether to use full distance estimation, that is, estimating distances from
the center of the point count or from the transect line, to all birds heard or seen,
or to use estimation within distance bands or belts. In the latter case, one needs
to decide on the specific distance bands.
We would always recommend recording some measure of the distance to each
bird seen or heard because this provides a useful measure of bird detectability
38 |Bird census and survey techniques